This Perspective describes the diverse roles played by isocyanides in supporting photoluminescent transition metal complexes.

In this work, we introduce a series of cyclometalated iridium complexes and evaluate the suitability of this class of compounds in nonlinear optical (NLO) applications, with an emphasis on long-lived, panchromatic reverse-saturable absorption (RSA). The investigated complexes are represented by the general formula [Ir(C^N) 2 (CNAr dmp ) 2 ] + , (C^N = cyclometalating ligand, CNAr dmp = 2,6-dimethylphenyl isocyanide). Seven such complexes were synthesized and characterized, including in-depth analysis of their photophysical properties (UV−vis absorption, photoluminescence, and transient absorption). This series of compounds contains seven different cyclometalating ligands (2-phenylbenzothiazole (bt Ir6), and 6-nitro-2,4-diphenylquinoline (NO 2 dpq) (Ir7)), which have profound effects on their ground-state and excited-state absorption spectra. To evaluate the effects of the isocyanide ancillary ligands, some heteroleptic bis-cyclometalated iridium(III) acetylacetonate (acac) analogue complexes are included as points of comparison. In the ground state, the bis-isocyanide complexes display UV−vis absorption with the characteristic 1 LC (π → π*) band at λ < 350 nm and 1 MLCT bands at 350−500 nm. Five of the complexes (Ir1, Ir2, Ir4, Ir5, and Ir6) display broad, intense triplet excited-state absorption with no ground-state bleach (GSB) over the spectral window of 400−900 nm, with excited-state lifetimes spanning three orders of magnitude from ∼32 ns to 12 μs. The photophysical data suggests that the isocyanide ancillary ligand blue-shifts the GSB transient into the UV, which is normally found in the middle of the visible region for cyclometalated iridium complexes. This study demonstrates the applicability of cationic cyclometalated iridium(III) bis-isocyanide complexes as candidate RSA materials.

Here, we demonstrate facile [4 + 4] coordination-driven self-assembly of cyclometalated iridium­(III) using linear aryldiisocyanide bridging ligands (BLs). A family of nine new [Ir­(C^N)2(μ-BL)]4 4+ coordination cages is described, where C^N is the cyclometalating ligand2-phenylpyridine (ppy), 2-phenylbenzothiazole (bt), or 1-phenylisoquinoline (piq)and BL is the diisocyanide BL, with varying spacer lengths between the isocyanide binding sites. These supramolecular coordination compounds are prepared via a one-pot synthesis, with isolated yields of 40–83%. 1H NMR spectroscopy confirms the selective isolation of a single product, which is affirmed to be the M4L4 square by high-resolution mass spectrometry. Detailed photophysical studies were carried out to reveal the nature of the luminescent triplet states in these complexes. In most cases, phosphorescence arises from the [Ir­(C^N)2]+ nodes, with the emission color determined by the cyclometalating ligand. However, in two cases, the lowest-energy triplet state resides on the aromatic core of the BL, and weak phosphorescence from that state is observed. This work shows that aromatic diisocyanide ligands enable coordination-driven assembly of inert iridium­(III) nodes under mild conditions, producing supramolecular coordination complexes with desirable photophysical properties.

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Our group has developed cyclometalated iridium complexes as nonlinear optical materials, focusing especially on reversesaturable absorption (RSA). Cationic cyclometalated iridium complexes with isocyanide ancillary ligands offer several advantages in this context, elaborated in previous studies. This talk describes next-generation complexes of the general formula [Ir(C^N) 2 (CNAr) 2 ] + , where C^N is a variable cyclometalating ligand and CNAr is a pyrene-decorated aryl isocyanide. In these compounds the dominant ground-state absorption transitions, especially in the visible range, are controlled by the C^N ligand. However, the lowest-energy triplet excited state (T 1 ) is typically located on the pyrene moiety, which has two consequences on the spectroscopic properties. First, these compounds exhibit temperaturedependent luminescence profiles. At room temperature, photoluminescence is mostly quenched by triplet energy transfer to the pyrene, and only residual pyrene fluorescence is observed. At low temperature (77 K), phosphorescence from the pyrene is turned on, and bright red luminescence is observed. The pyrene isocyanides also have profound impacts on the transient absorption spectroscopy of these compounds. Following visible excitation, a strongly absorbing, long-lived excited state is rapidly populated, which gives rise to ESA over the entire visible range and is assigned to the pyrene triplet state. The pyrene isocyanide complexes have higher excited-state absorption cross section (i.e. larger ΔOD) relative to first-generation complexes, and the excited-state lifetime increases by as much as an order of magnitude.